Resolves based on the number under "Lifetime members" at https://www.givingwhatwecan.org/about-us/members, which is 8,850 as of market creation on 28th April 2024.
Fake pledges or other attempts at manipulation won't be counted, according to the best of my judgment.
I think the latter only started up in the last year or two; my sense is that GWWC, while having been around for a long time, recently went through some management changes/started staffing up and I now think they're live players in the space. So I wouldn't be that surprised if they do manage to drive many more pledges!
Relevantly, this is an explicit goal that they have, and they've recently started launching partnerships with other organisations with a goal of accelerating their growth. See e.g. https://forum.effectivealtruism.org/posts/uZzXRyAwkDHLfu94W/we-ve-renamed-the-giving-what-we-can-pledge